1. Field of the Invention
The present invention relates to an optical pickup apparatus.
2. Description of the Related Art
There is known an optical pickup apparatus including a laser light source that can selectively emit laser light having a wavelength for DVD (Digital Versatile Disc) and laser light having a wavelength for CD (Compact Disc), for example. The laser light source has two light emitting points respectively emitting these two laser lights, with a predetermined space between the points.
This optical pickup apparatus including two types of diffraction gratings that generate 0th order light and ±1st order diffracted lights for generating tracking error signals for each of the two laser lights. Specifically, the two types of the diffraction gratings are assembled so that grating surfaces thereof having different configurations are arranged back to back (See Japanese Patent Laid-Open Publication No. 2007-164962, for example) or assembled so that they are opposed to each other (See Japanese Patent Laid-Open Publication No. 2007-149249, for example).
For example, a diffraction grating provided for laser light having a wavelength for DVD has a configuration for generating the 0th order light and ±1st order diffracted lights suitable for an in-line differential push-pull method by which a tracking error signal is generated, while a diffraction grating provided for laser light having a wavelength for CD has a configuration for generating the 0th order light and ±1st order diffracted lights suitable for a three-beam differential push-pull method by which a tracking error signal is generated (See Japanese Patent Laid-Open Publication No. 2004-145915, for example).
That is, since these two types of the diffraction gratings have the configurations different from each other, they are assembled so that the two grating surfaces are arranged back to back or so that they are opposed to each other as mentioned above, to be incorporated as a single diffraction grating in an optical pickup apparatus for both DVD and CD.
In the above-mentioned optical pickup apparatus, since the laser light having a wavelength for DVD, for example, from the laser light source is applied to both of the above-mentioned diffraction gratings for DVD and CD, the laser light is affected by the diffraction grating for CD.
For example, when performing tracking error control in reproduction, etc., of DVD, in order to generate a differential push-pull signal, it is necessary to diffract the laser light having a wavelength for DVD with the diffraction grating for DVD, to generate the ±1st order diffracted lights (±1st order diffracted lights for DVD). At the same time, the 0th order light is generated from the laser light having a wavelength for DVD by the diffraction grating for DVD, however, the ±1st order diffracted lights (noise) generated by diffracting this 0th order light with the diffraction grating for CD have the same levels in intensity as those of the above-mentioned ±1st order diffracted lights for DVD. This leads to a drop in accuracy of the tracking error signal.